Linéarisation numérique et annulation du couplage capacitif pour un microscope à effet tunnel

Abstract

The development and test of a DSP-based digital control loop to control the tip-to-specimen distance in a Scanning Tunneling Microscope are described in this thesis. There are two difficulties normally associated with traditional analog controllers: One is the exponential relationship between the tip-to-specimen distance and the tunnel current, the other is the capacitive coupling between the electrodes of the scan tube and the input of the current-to-voltage converter, which induces noise in the control loop as well as in the scanning image.The digital controller we designed tackles both problems. It provides a frequency-independent linearization of the tunnel junction by inserting a digital logarithm function in the control loop; A precise cancellation of the capacitive coupling in the Z direction is also achieved by first identifying its transfer function using a LMS adaptive filter, then compensating its effect in the digital domain using the identified model. Added benefits of the completely digital control system are: (1) Easy adjustment of the control parameters through simulation with great precision, decreasing the possibility of crashing the tip to the specimen. (2) Stable control parameters not likely to drift with time or temperature. (3) Easy on-line calibration of the current detection offset bias. (4) Easy automation of the instrument. (5) No need of additional digitizing electronics, since the control loop already does the digitization, and operates in the digital domain. In the described system, the control loop is implemented on an inexpensive fixed-point DSP TMS320C50 board.The sampling rate could reach 25 KHz. Simulations and practical results are presented.